1. Field of the Invention
This invention relates to a molded product of polypropylene having a low coefficient of linear expansion and a good balance of mechanical properties.
2. Description of the Related Arts
Polypropylene is a thermoplastic resin which is excellent in bending strength, modulus of elasticity in bending and other mechanical properties, and in chemical resistance, and yet is inexpensive, and is, therefore, used for making a wide range of molded products. It is used to make, for example, various molded parts forming, or attached to, the exterior of an automobile, or vessel, such as a side garnish 1, an elongated side molding 2 and bumper 3, which are shown in FIGS. 4 to 6, respectively, of the accompanying drawings.
Those parts are, however, likely to undergo a high degree of thermal expansion and contraction, as they are exposed to a widely varying outdoor temperature. For example, the variation in temperature between about 10.degree. C. and about 40.degree. C. produces a difference of about 3 mm per meter in the length of any such part. The expansion and contraction of such a part is likely to cause deterioration of the molding appearance, i.e. reduction of the clearance accuracy, surge, etc. The great likelihood of such a part expanding and contracting means also that the part will come to lack dimensional stability relative to its base. The lack of dimensional stability prevents increased use of polypropylene for making not only any of the parts mentioned, but also a variety of other molded products.
Attempts have, therefore, been made to add talc, calcium carbonate, mica, etc. to polypropylene in order to make a molded polypropylene product with improved dimensional stability. None of these attempts has, however, produced a satisfactory result. Japanese Patent Publication No. 15824/1981 proposes a composition which comprises polypropylene, a styrene-butadiene block copolymer, and particles of calcium carbonate treated with a fatty acid ester. This composition does not, however, have a coefficient of linear expansion which is low enough to be satisfactory. Moreover, this composition is relatively low in weatherability, because the butadiene forming the copolymer has carbon double bonds (C.dbd.C).